1. Field of the Invention
The invention relates to an electrophotographic color image forming apparatus for use in color coping machines, printers, facsimiles, and other apparatuses.
2. Related Art
A color image forming apparatus such as described below has heretofore been known as disclosed in U.S. Pat. No. 3,838,919. The color image forming apparatus includes: a first transfer means that sequentially transfers toner images developed on an image carrying body to an intermediate transfer body; a second transfer means that collectively transfers the toner images superimposed on an intermediate transfer body onto a recording medium; and a cleaning means such as a cleaning blade that removes a toner remaining on the intermediate transfer body. Further, it is known to use as the intermediate transfer body either a beltlike member which has a predetermined electric resistance and which is stretched by a group of rollers including a drive roller from inside, or a hard roller that has an electric resistance layer over the surface of a metal base.
In such image forming apparatus, after the collective transfer of the toner images onto the recording medium from the intermediate transfer body by the second transfer means (hereinafter referred to as the "secondary transfer process"), the cleaning blade comes in contact with the intermediate transfer body, and it is before the formation of the first-color toner image of a next print image that the cleaning blade moves away from the intermediate transfer body. Still further, in order to sequentially transfer the toner images from the image carrying body to the intermediate transfer body by the first transfer means (hereinafter referred to as the "primary transfer process"), a technique disclosed in Unexamined Japanese Patent Publication No. Hei. 7-77880 has been employed. This technique is designed to bring the cleaning blade into contact with the intermediate transfer body after all the color toner images to be superimposed have been transferred thereto.
However, in the above-identified conventional example in which the cleaning blade is not only brought into contact with the intermediate transfer body after the second transfer process has been completed, but also moved away from the intermediate transfer body before the first-color toner image of a next print image is formed, as well as in the conventional example disclosed in Unexamined Japanese Patent Publication No. Hei. 7-77880, the primary transfer process must not be performed while the cleaning blade is in contact with the intermediate transfer body. That is, it is necessary to suspend the primary transfer process of the first-color toner image of a next print image while the last-color toner image is moving along the intermediate transfer body by at least a distance two times or more the circumferential length of a maximum print image from the start of the primary transfer process of such last-color toner image of the previous print image in a constant-speed continuous printing operation. This has imposed the problem of reducing the print speed. Also known is a technique of switching the process speed while the primary or the secondary transfer process is not being performed in order to improve the print speed. However, this technique makes the apparatus large in structure and elevates the cost of manufacture. Still further, even if a high speed process is introduced, there have been image defects attributable to fluctuations in loads applied to the intermediate transfer belt, such as displacements caused when one color toner image is superimposed upon another at the time of primary transfer process.